Conventional steel components such as gears and pulleys of automobiles or industrial machinery are made by using, as starting materials, rolled steel bars or wire rods of alloy steel for mechanical structures such as SCr420, SCM 420, and SNCM 420 specified by JIS standard, which are roughly formed through hot forging or cold forging. After normalized if necessary, the roughly formed rolled steel bars or the wire rods are machined, and then carburizing-quenched or carbonitriding-quenched, and thereafter are tempered at a temperature of not more than 200° C. The rolled steel bars or the wire rods are further subjected to shot peening processing if necessary for production, thereby securing a property required for each component such as contact fatigue strength, bending fatigue strength, and wear resistance.
Due to recent progress in weight reduction and size reduction of components in order to achieve improvement of fuel efficiency of automobiles and high output performance of engines, load applied to the components tends to be increased. Meanwhile, in the light of cost reduction, there are also strong needs to omit additional surface processing such as shot peening after carburizing-quenching. There are also strong needs to enhance machinability because the percentage of machining cost in the total processing cost of components is great.
It is common to add more alloying elements in order to enhance fatigue strength of components, but this often deteriorates machinability. Hence, it has been desired to cope with both bending/contact fatigue strength and machinability of the components at a high level.
The aforementioned “contact fatigue” includes “surface fatigue”, “linear fatigue”, and “point fatigue”, but there barely occur a “linear” contact or a “point” contact in reality; thus the “surface fatigue strength” is handled as the contact fatigue strength.
Pitching is one of fracture morphologies of the surface fatigue, and the damage morphology of the surface fatigue caused on a surface tooth of a gear or a pulley, etc., is chiefly pitching. Hence, enhancement of the pitching strength corresponds to enhancement of the aforementioned surface fatigue strength, and thus the “pitching” will be described as the “surface fatigue”, and the “pitching strength” is referred to as the “surface fatigue strength”, hereinafter.
JP60-21359A, JP7-242994A, JP7-126803A suggest improvement of steel for gears. Specifically, JP60-21359A discloses steel for gears specified to contain Si: not more than 0.1% and P: not more than 0.01% so as to provide gears excellent in strength and stiffness, and having high reliability. JP7-242994A discloses steel for gears, gears, and a method of producing the gears specified to contain Cr: 1.50 to 5.0%, and 7.5%>2.2×Si(%)+2.5×Mn(%)+Cr(%)+5.7×Mo(%) if necessary, or Si: 0.40 to 1.0% so as to be excellent in tooth surface strength. JP7-126803A discloses carburized steel for gears specified to contain Si: 0.35 to not more than 3.0%, and V: 0.05 to 0.5% so as to be preferable to provide gears excellent in wear resistance and surface fatigue strength as well as bending fatigue strength.